The present invention relates to a control apparatus of a variable delivery pump for driving an actuator by controlling a delivery flow rate and a delivery pressure of the variable delivery pump and, more particularly, to a control apparatus of a variable delivery pump in which when the actuator which is moving at a constant velocity by the control of the delivery flow rate stops, the control mode is switched to the delivery pressure control mode, thereby keeping the stop pressure constant.
As a conventional control apparatus of a variable delivery pump, for example, there is known an apparatus as shown in FIG. 1.
In FIG. 1, reference numeral 10 denotes a variable delivery pump in which an angle of a slant plate (swash plate) by changing, e.g., an input voltage or input current, so that a piston stroke changes and a delivery amount can be changed.
Reference numeral 12 denotes a servo amplifier to control a flow rate; 14 indicates a servo amplifier to control a pressure; 16 a switch to change over outputs of the servo amplifiers 12 and 14; 18 a voltage comparator to control the switching operation of the switch 16; and 20 a cylinder as a control load.
In such a control apparatus, the switch 16 is set to the A size in the ordinary flow rate control mode. An angle sensor 28 to detect an angle of inclination of the slant plate is attached to the pump 10. A detection angle .theta. of the angle sensor 28 represents a pump delivery flow rate Q. The detection angle .theta. of the angle sensor 28, namely, the delivery flow rate Q is input to an adding point 22. A designated flow rate value Q.sub.c is also input to the adding point 22 from a flow rate setting device (not shown). Therefore, the difference between the designated flow rate value Q.sub.c and the pump delivery flow rate Q is obtained by the adding point 22 and input to the servo amplifier 12. Thus, the pump 10 is controlled in a feedback manner by an output of the servo amplifier 12 so as to keep the pump delivery flow rate Q to the designated flow rate value Q.sub.c. A piston rod 32 of the cylinder 20 is moved at a constant velocity.
When the piston rod 32 collides with an object 100 by the actuation of the cylinder 20 due to such a flow rate control and mechanically stops, the pressure in the cylinder suddenly starts increasing.
A difference value (P.sub.c -.alpha.) which is derived by subtracting a predetermined value .alpha. from a designated pressure value P.sub.c which is output from a pressure setting device (not shown) is input as a reference value to the voltage comparator 18. A pump delivery pressure P detected by a pressure sensor 30 is also input to the voltage comparator 18. Therefore, when the pump delivery pressure P which has started increasing by the collision of the piston rod 32 with the object 100 reaches the reference value (P.sub.c -.alpha.) which is slightly smaller than the designated pressure value P.sub.c, an output of the voltage comparator 18 is inverted, so that the switch 16 is switched to the B side. By the connection of the switch 16 to the B side, the difference (P.sub.c -P) from an adding point 26 is input to the servo amplifier 14. There is performed a pressure control to control a delivery amount of the variable delivery pump 10 so as to keep the pump delivery pressure, i.e., the stop pressure P to the designated pressure value P.sub.c by the output of the servo amplifier 14.
However, such a conventional control apparatus of the variable delivery pump has the following problems.
Since the switching from the flow rate control to the pressure control is performed when the delivery pressure P reaches the reference value (P.sub.c -.alpha.) which is slightly smaller than the designated pressure value P.sub.c, for example, as shown in FIG. 2, when the designated flow rate value Q.sub.c is set to a large value or when it reaches the stroke end, if the cylinder volume is small, even if the control mode is switch from the flow rate control to the pressure control, the fluid of the amount larger than it is needed is discharged due to the response time lag of the pump. Thus, an extremely large overshoot .DELTA.P is generated for the designated pressure value P.sub.c.
To prevent this, if the gain of the servo amplifier 14 is set to a very low value in order to reduce the overshoot .DELTA.P, the settling time until the stop pressure is settled to the designated pressure value P.sub.c becomes long, causing a problem of deterioration in control accuracy.
On the other hand, when the piston rod 32 collides with the object 100 and the cylinder 20 stops, the control mode is switched to the pressure control. However, in the pressure control, the product (V.beta.) of a content volume V of the pipes of the cylinder 20 and a compressibility .beta. of the operating fluid exerts a large influence on the pressure control.
Therefore, in the conventional control apparatus, in the state in which the control apparatus is assembled to the variable delivery pump 10, the gain of the servo amplifier 14 is adjusted so as to obtain the optimum control characteristics while observing the detection waveform of the pressure sensor 30 upon switching to the pressure control.
However, the adjusting work to obtain the optimum pressure characteristic is extremely complicated.
Further, in order to eliminate the overshoot when switching to the pressure control as shown in FIG. 2, it is necessary to adjust a predetermined value .alpha. for deciding a reference voltage of the voltage comparator 18 to the optimum value.
For example, when the predetermined value .alpha. is set to a large value to eliminate the overshoot, if the delivery pressure fluctuates due to the disturbances of the resistance and the like of the fluid passing through the pipes at the time of the flow rate control, there is caused a malfunction such that the pump delivery pressure P exceeds the reference value (P.sub.c -.alpha.) and the control mode is switched to the pressure control. The occurrence of the malfunction such as to switch to the pressure control during the flow rate control raises the pump delivery pressure to the designated pressure value P.sub.c, so that the delivery flow rate suddenly increases. There is a fear such that the piston rod 32 of the cylinder 20 moves at an abnormal high speed.
On the contrary, when the predetermined value .alpha. is set to a small value, after the pump delivery pressure P was settled to the designated pressure value P.sub.c due to the switching to the pressure control, if the designated pressure value P.sub.c is reset to a low value in order to reduce the force for pushing the piston rod 32 of the cylinder 20, an undershoot occurs due to the pressure control to reduce the pump delivery pressure. When the pump delivery pressure decreases to the reference value (P.sub.c -.alpha.) or less by the undershoot, the control mode is switched to the flow rate control. Thus, the pump delivery pressure abnormally rises due to the flow rate control and an overshoot occurs. The control mode is again switched to the pressure control by the overshoot, so that an undershoot is caused. The repetition of such overshoot and undershoot causes an abnormal operation of the hunting of the pump delivery pressure.
Consequently, it is extremely difficult to adjust the predetermined value .alpha. for switching from the flow rate control to the pressure control to the optimum value and there is a problem such that the reliability of the apparatus cannot be perfectly guaranteed.